Analog modulation of optical signals

ABSTRACT

An optically modulated signal is generated for use in transporting data by generating a so-called sub-carrier modulated optical signal and, then, vector modulating the sub-carrier modulated optical signal to yield the desired modulated optical signal for transmission. The vector modulation includes a phase component and an amplitude component. In one specific embodiment of the invention, an apparatus for use in generating a modulated optical signal includes a generator to generate a sub-carrier modulated optical signal including an optical carrier and at least one sub-carrier and an analog vector modulator coupled to receive both the sub-carrier modulated optical signal from the generator and a data signal. The analog vector modulator generates an output optical signal by phase modulating and/or amplitude modulating the sub-carrier of the received optical signal in response to the data signal.

TECHNICAL FIELD

This invention relates to optical transmission systems and, moreparticularly, to analog modulation of optical signals.

BACKGROUND OF THE INVENTION

There is a clear trend towards using higher carrier frequencies and highdate rates in optical communications systems. One such arrangementemploys Mach Zehnder modulators supplied with individual light, i.e.,optical carrier, signals to be modulated. Each of the Mach Zehndermodulators is modulated directly by a microwave sub-carrier that hasbeen modulated with the desired digital data. See for example, anarticle authored by W. D. Jemison et al., entitled “Microwave PhotonicVector Modulator for High-Speed Wireless Digital Communications”, IEEEMicrowave And Wireless Components Letters, pp 125–127, Vol. 12, No. 4,April 2002. This and other known optical signal modulation arrangementshave limited bandwidth and, hence, there is a limit on the data ratethat may be used in optical communications systems employing suchmodulation schemes.

SUMMARY OF THE INVENTION

These and other problems and limitations of prior known opticalmodulation arrangements are overcome in applicants' unique invention bygenerating a so-called sub-carrier modulated optical signal and, then,vector modulating the sub-carrier modulated optical signal to yield thedesired modulated optical signal for transmission. The vector modulationincludes a phase component and/or an amplitude component.

In one specific embodiment of the invention, the vector modulation iseffected by splitting the sub-carrier modulated optical signalcomprising a carrier and an at least one unmodulated sub-carrier into aplurality of similar optical signals, phase modulating the carrier ofeach of the similar optical signals with the data and delaying theresulting optical signals in a prescribed manner relative to oneanother. Then, the “delayed” signals are combined to yield the signalcomprising the vector modulated sub-carrier modulated optical signal tobe transmitted. Wherein a signal can be “delayed” by a zero (0) delayinterval.

In another embodiment of the invention, the “sub-carrier modulatedoptical signal” is obtained by using two optical carriers that areslightly offset in wavelength relative to each other so as to generate abeating tone in a remote photodiode detector. The two optical carriersare coupled together and supplied to the vector modulator to be vectormodulated with the data signal.

In yet another embodiment of the invention, the phase modulators in thevector modulator are grouped in sets. Then, the phase modulated opticalsignals in each set are each delayed by the same interval for that set.The delay intervals for the individual sets are in a prescribedrelationship to each other. In one example, a first set has zero delayinterval, a second set has a delay interval of τ, a third set has adelay interval of 2τ, a fourth set has a delay interval of 3τ and so on.

In still another embodiment of the invention, the data signal isprecoded to generate signals to drive particular ones of the phasemodulators of the vector modulator to desired vector modulation statesfor each state of the data signal.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows, in simplified block diagram form, one embodiment of theinvention;

FIG. 2 also shows, in simplified block diagram form, a second embodimentof the invention;

FIG. 3 illustrates, in simplified block diagram form, a third embodimentof the invention;

FIG. 4 shows, in simplified block diagram form, details of a vectormodulator that may be employed in the embodiments of the invention;

FIG. 5 shows, in simplified block diagram form, details of a specificvector modulator that may be employed in the embodiments of theinvention;

FIG. 6 shows a constellation plot showing states of the vector modulatorof FIG. 5;

FIG. 7 illustrates, in simplified block diagram form, a simplifiedversion of the invention to illustrate operation of the precoder anddriver 201 shown in FIGS. 2 and 3;

FIG. 8 is a table useful in explaining the operation of precoder anddriver 201; and

FIG. 9 is a constellation plot showing the states of the modulator shownin FIG. 7.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 1 shows, in simplified block diagram form, one embodiment of theinvention. Specifically, shown is optical light source 101 typicallyincluding a laser to generate an optical carrier signal at a desiredwavelength. Exemplary optical carrier signals to be processed haveoptical frequencies of about 2.3×10¹⁴ Hertz to about 1.8×10¹⁴ Hertz,i.e., a wavelength of about 1.3 microns to about 1.7 microns. In oneexample, an optical carrier signal having a wavelength of approximately1.55 microns, i.e., a frequency of 1.93×10¹⁴ Hertz is generated by lightsource 101 and supplied to amplitude modulator 102. Also supplied toamplitude modulator 102 via input 103 is a desired amplitude modulationsignal that modulates the optical carrier from source 101 to obtain asub-carrier modulated optical signal. As is known, the spectrum of thissignal includes the optical carrier and two sidebands, i.e.,sub-carriers. This modulation may be achieved in a number of ways. Itcan be realized by use of an electro-optical modulation scheme througheither direct modulation of the laser in source 101 or by an externalmodulation using, for example, a Mach Zehnder modulator or anelectro-absorption modulator. The frequency of the modulation signal isin the microwave/millimeter-wave range. Another approach using twooptical carrier signals is described below in conjunction with theembodiment of FIG. 3. The sub-carrier-modulated optical signal fromamplitude modulator 102 is supplied to vector modulator 104 where thesubcarrier is phase and/or amplitude modulated, i.e., vector modulatedwith a data signal supplied via input 105 to yield the optical transportsignal at output 106. In this embodiment, the data rate can be as highas the sub-carrier frequency. Embodiments of vector modulators 104 thatmay be employed in the embodiment of the invention of FIG. 1 are shownin FIGS. 4 and 5 and described below.

FIG. 2 also shows, in simplified block diagram form, a second embodimentof the invention. The embodiment of FIG. 2 is similar to that of FIG. 1and includes similar elements that are essentially identical from both aphysical and functional standpoint. These similar elements have beensimilarly numbers as those in FIG. 1 and will not be discussed again indetail. The only significant difference between the embodiments of FIG.1 and FIG. 2 is the use of Precoder and Driver 201. As can be seen, oneor more data signals are supplied to precoder and driver 201 via inputconnections 105-1 through 105-N. In precoder and driver 201, the datasignals are employed to code and generate voltage signals for driving,phase modulators in vector modulator 104. These voltage signals aresupplied via output connections 107-1 through 107-M to vector modulator104. Again, details and operation of vector modulator 104 are describedbelow regarding FIGS. 4 and 5 and a simple example of operation ofprecoder and driver 201 in conjunction with vector modulator 104 isdescribed below regarding FIG. 7.

FIG. 3 illustrates, in simplified block diagram form, a third embodimentof the invention. The embodiment of FIG. 3 is similar to that of FIG. 2and includes similar elements that are essentially identical from both aphysical and functional standpoint. These similar elements have beensimilarly numbers as those in FIG. 2 and will not be discussed again indetail. The only significant difference between the embodiments of FIG.2 and FIG. 3 is how the sub-carrier modulated optical signal isgenerated in the embodiment of FIG. 3. As indicated above, there areseveral ways of generating the desired sub-carrier modulated opticalsignal upon which the data signal will eventually be modulated. In theembodiment of FIG. 3, a coherent technique is advantageously employed torealize the desired sub-carrier modulated optical signal. Specifically,in this example, two carrier optical signals are generated by employingtunable laser light sources 301 and 302. Signals for controlling thefrequency (wavelength) of the laser light are supplied via connections303 and 304 to tunable light sources 301 and 302, respectively. Theresulting two optical carrier signals are tuned to have a prescribedwavelength offset and are overlaid coherently. At a receiving detector,typically a photodiode, the difference in wavelength generates a beatingsignal. The lasers, in this example, are single mode tunable lasers.This allows very wide tunability, since tuning the wavelength of onelaser directly changes the frequency of the beating signal. Anotheradvantage of using the two tunable lasers is that the resulting opticalsynthesizer generates a virtually harmonic free optical source. Thisresults in an improved dynamic range and spurious free dynamic range ofthe optical link. Then, the two optical carriers that are offsetrelative to each other are supplied to optical coupler 305 where theyare combined, i.e., mixed, to form the desired o sub-carrier modulatedoptical signal, which is supplied to vector modulator 104.

FIG. 4 shows, in simplified block diagram form, details of one vectormodulator 104 that may be employed in the embodiments of the invention.The optical vector modulator 104 is based on the summing of multipleoptical tapped delay lines. The principle of operation is as follows:The to be phase-shifted and/or to be amplitude-modulated signal couldeither be an amplitude-modulated single optical carrier generated byamplitude modulator 102 (FIG. 1 or FIG. 2) or include two opticalcarriers that are slightly offset in wavelength to generate a beatingsignal generated by tunable light sources 301 and 302 and coupler 305 ofFIG. 3. This sub-carrier modulated optical signal is, when fed via 401into the optical vector modulator 104 of FIG. 4, split into a pluralityof similar branches by a power splitter 402, e.g., a multimodeinterference (MMI) coupler. Each branch is equipped with a phasemodulator 403-1 through 403-M to adjust the phase of the optical carrierfollowed by an optical delay line. The delay in series with phasemodulator 403-1, in this example, is zero (0). The delays in theremaining branches including phase modulators 403-2 through 403-M aregenerated by delay units 404-1 through 404-(M−1), respectively. Each ofthese delay lines in delay units 404-1 through 404-(M−1) changes thephase of the sub-carrier of the optical signal from phase modulators403-2 through 403-M, respectively, by a fixed amount. For example, thedelay line in unit 404-1 provides a delay of τ, delay unit 404-2provides a delay of 2τ and delay unit 404-(M−1) provides a delay of(M−1)τ. Typically, a delay τ of 1/(M*sub-carrier frequency) is required.Thus, if the sub-carrier frequency is 40 GHz, the total delay rangeshould be 0, . . . 25 picoseconds (ps). Another MMI coupler, i.e., powercombiner 405, combines all of the phase-shifted and delayed opticalsignals from all branches to produce a modulated output optical signalat output 106, which will interfere constructively or destructivelydepending on the summing optical phases from all tributary branches.Therefore, by interfering signals with different carrier phase, thephase and the amplitude of the carrier of the summing signal can be setto an arbitrary state. These interfered optical carriers will producemicrowave phasors with prescribed amplitude and phase at the remoteoptical detector.

The phase modulator 403 of each branch can be fabricated e.g. in amaterial system with linear electro-optic effect, as InP, GaAs orLiNbO₃. The effective refractive index of an optical waveguide changesin proportion to the applied electrical field perpendicular to thiswaveguide. A high frequency distributed electrical waveguide isengineered to co-propagate with the optical wave with matchedpropagating velocity to deliver the local electrical field with highmodulation bandwidth. The different branches will delay the opticalsignal by a different length of time. This results in differentsub-carrier phases at the outputs of these delay lines in units 404. Inthe combiner 405, these different output signals that interfereconstructively have a different sub-carrier phase due to the differenttime delays these signals experienced. The sub-carrier of the signalafter the MMI coupler, i.e., power combiner 405, is the sum of allsub-carriers of the signals that interfere constructively.

FIG. 5 shows, in simplified block diagram form, details of a specificvector modulator that may be employed in the embodiments of theinvention. Such an optical vector phase modulator 104 can be realized ina number of different configurations. One of the simplest designs is touse eight branches, which are grouped into pairs of two with the samedelay line length. Basically, the branch pairs form Mach-Zehndermodulators 403. The delay lengths of these different pairs are designedsuch that they differ by a quarter period of the center frequency of theenvelope signal. If now only one of the Mach-Zehnder structures isbiased such that the optical signal of each arm interferesconstructively, one can set four different phase states each separatedby 90°. If two neighboring Mach Zehnder phase modulators 403 areswitched on together, this results in a phase offset of 45° incomparison to the signal from a single Mach Zehnder phase modulator 403.

Specifically, FIG. 5 shows input 401 to which an appropriate sub-carriermodulated optical signal is supplied, for example, from amplitudemodulator 102 of FIG. 2 or the two combined offset optical carriersgenerated by tunable light sources 301 and 302 and coupler 305 of FIG.3. The supplied signal is split into eight branches and a branch issupplied to each one of phase modulators 403-1 through 403-8. Again, thephase modulators 403-1 through 403-8 are grouped into sets of two andeach set has a different delay path. For example, a first set includingphase modulators 403-1 and 403-2, has a zero delay; a second setincluding phase modulators 403-3 and 403-4, has a delay of τ generatedby delay lines in delay units 501-1 and 501-2; a third set includingphase modulators 403-5 and 403-6, has a delay of 2τ generated by delaylines in delay units 501-3 and 501-4; and a fourth set including phasemodulators 403-7 and 403-8, has a delay of 3τ generated by delay linesin delay units 501-5 and 501-6. The optical signals from the eightbranches are combined in power combiner 405 to yield the desired outputoptical signal at output 106.

FIG. 6 shows all possible phase/amplitude states of the optical envelopeproduced by vector modulator 104 of FIG. 5, if each phase modulator isdriven either with 0V or with a Voltage that results in a π/2 phaseshift of the optical carrier or with a Voltage that results in a π phaseshift of the optical carrier. It is possible to use digital, multi-levelor analog control signals, which allows obtaining all possiblephase/amplitude states in-between the discrete ones shown in FIG. 6.

FIG. 7 illustrates, in simplified block diagram form, a simplifiedversion of the invention to illustrate operation of the precoder anddriver 201 shown in FIGS. 2 and 3. Specifically, optical light source101 supplies an sub-carrier modulated optical signal to the vectormodulator 104 and, therein, to power splitter 402. In this example,power splitter 402 splits the supplied optical carrier into three equalversions, i.e., branches, each of which is supplied to one of phasemodulators 403-1, 403-2 and 403-3. An output of phase modulator 403-2 issupplied to delay unit 404-1 wherein a delay line delays the phasemodulated signal by τ. An output of phase modulator 403-3 is supplied todelay unit 404-2 wherein a delay line delays the phase modulated signalby 2τ. In turn, the output from phase modulator 403-1 and the outputsfrom delay units 404-1 and 404-2 are supplied to power combiner 405where they are combined, as described above, to generate the desiredtransport output optical signal at output 106. The transport opticalsignal is transmitted to a remote receiver where it is detected via, forexample photodiode 701, which generates a desired signal at terminals702. Returning, phase modulators 403-1, 403-2 and 403-2 are suppliedcontrol signals via paths 107-1, 107-2 and 107-3, respectively, fromprecoder and driver 201. The operation of precoder 201 can be explainedby referring to the table in FIG. 8. In this example, it is desired togenerate symbols A, B, C and D. This is realized by supplying a binaryinput to terminal 105-1 and 105-2 of precoder and driver 201. As seen inFIG. 9, the desired vectors corresponding to the location of the symbolsA, B, C and D in the constellation shown in FIG. 9 are (0,0) 1<60°,(0,1) 1<−60°, (1,0) 1<180° and (1,1) 0, respectively. These vectors arerealized by controlling phase modulators 403-1, 403-2 and 403-3 by themodulation voltages V₁, V₂ and V₃, respectively. Thus, for symbol A(binary input 0,0) V₁=V₂=V₃=0, for symbol B (binary 0,1) V₁=V_(π) andV₁=V₂=0, for symbol C (binary 1,0) V₁=V₂0 and V₃=V_(π) and for symbol D(binary 1,1) V₁=V₃=0 and V₂=V_(π).

The above-described embodiments are, of course, merely illustrative ofthe principles of the invention. Indeed, numerous other methods orapparatus may be devised by those skilled in the art without departingfrom the spirit and scope of the invention.

1. Apparatus for use in generating a modulated optical signalcomprising: a generator to generate a sub-carrier modulated opticalsignal including an optical carrier and at least one sub-carrier; and ananalog vector modulator coupled to receive both said sub-carriermodulated optical signal from the generator and a data signal andconfigured to generate an output optical signal by phase modulatingand/or amplitude modulating the sub-carrier of the received opticalsignal in response to said data signal, said analog vector modulatorincluding an optical power splitter, a plurality of optical branches,and an optical power combiner, the splitter being configured to splitsaid optical sub-carrier signal into a plurality of optical branchsignals and to supply each optical branch signal to a corresponding oneof said optical branches, each of said optical branches including anoptical phase modulator for phase shifting its supplied optical branchsignal.
 2. The apparatus as defined in claim 1 wherein said generator ofsaid sub-carrier modulated optical signal includes a source of theoptical carrier signal, an optical amplitude modulator and a device tosupply a baseband signal to said optical amplitude modulator, themodulator amplitude modulating said optical carrier signal to generatesaid sub-carrier modulated optical signal in response to receiving saidbaseband signal.
 3. The apparatus as defined in claim 1 wherein saidgenerator of said optical sub-carrier modulated optical signal includesat least a first tunable source of an optical carrier signal, a secondsource of an optical carrier signal source, the first source capable ofproducing the first optical carrier signal with a frequency that isoffset relative to a frequency of said second optical signal, and anoptical coupler for mixing said first optical carrier signal and saidsecond optical carrier signal to yield said sub-carrier modulatedoptical signal.
 4. The apparatus as defined in claim 1 wherein saidanalog vector modulator further includes an optical delay for delayingthe corresponding optical branch signal, the power combiner beingconfigured to combine said plurality of delayed optical phase shiftedsignals to generate said output optical carrier signal.
 5. The apparatusas defined in claim 4 wherein a delay interval produced by a Mth branchis (M−1), where M is the number of branches and M=1, 2 . . . M.
 6. Theapparatus as defined in claim 4 wherein said branches are grouped in aplurality of sets including at least two branches each and each branchin a set has a same delay interval.
 7. The apparatus as defined in claim6 wherein a delay interval of a Yth set of branches is (Y−1), where Y isthe number of sets and Y=1, 2 . . . Y.
 8. The apparatus as defined inclaim 1 wherein said splitter and said combiner are each a multimodeinterference coupler.
 9. The apparatus as defined in claim 1 furtherincluding a precoder to apply control voltages to said plurality ofoptical phase modulators in response to receiving said data signal. 10.The apparatus as defined in claim 9 wherein said data signal is adigital signal.
 11. A method for use in generating a modulated opticalsignal comprising: receiving a sub-carrier modulated optical signalincluding an optical carrier and at least one sub-carrier; and analogvector modulating said sub-carrier modulated optical signal with a datasignal to generate an output optical signal in which said onesub-carrier has been phase modulated and/or amplitude modulatedresponsive to said data signal, said analog vector modulating includingoptically power splitting said sub-carrier modulated optical signal intoa plurality of optical branch signals, supplying each optical branchsignal to a corresponding one of a plurality of optical branches,optically phase modulating each of said optical branch signals to phaseshift it.
 12. The method as defined in claim 11 further including thestep of generating said sub-carrier modulated optical signal bygenerating an optical carrier signal and optically amplitude modulatingsaid optical carrier with a prescribed signal to generate saidsub-carrier modulated optical signal.
 13. The method as defined in claim11 further including the step of generating said sub-carrier modulatedoptical signal by generating a first optical carrier signal, generatinga second optical carrier signal, and mixing said first optical carriersignal and said second optical carrier signal to yield said sub-carriermodulated optical signal, a frequency of said first optical carriersignal being offset relative to a frequency of said second opticalsignal.
 14. The method as defined in claim 11 wherein said step ofanalog vector modulating further includes delaying each of said phasemodulated optical branch signals by a prescribed delay interval and anoptically combining said plurality of delayed optical phase shiftedsignals to generate said output optical carrier signal.
 15. The methodas defined in claim 14 wherein a delay interval of a Mth branch is (M−1)where M is the number of branches and M=1, 2 . . . M.
 16. The method asdefined in claim 14 wherein said branches are grouped in a plurality ofsets including at least two branches each and each branch in a set has asame delay interval.
 17. The method as defined in claim 16 wherein adelay interval of a Yth set of branches is (Y−1), where Y is the numberof sets and Y=1, 2 . . . Y.
 18. The method as defined in claim 17wherein said data signal is a digital signal.
 19. The method as definedin claim 14 wherein said splitting and said combining are each realizedby employing a multimode interference coupler.
 20. The method as definedin claim 11 further including a step of precoding a supplied data signalto generate control voltages that cause said plurality of opticallyphase modulators to generate said phase shifted optical branch signals.